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The Mysterious Cosmic Strings

Cosmic-string loops wiggle and oscillate, producing gravitational waves, then slowly shrink as they lose energy until they disappear.
Cosmic-string loops wiggle and oscillate, producing gravitational waves, then slowly shrink as they lose energy until they disappear.
(Image: © Matt DePies/UW)

Paul M. Sutter is an astrophysicist at The Ohio State University, host of Ask a Spaceman and Space Radio, and author of "Your Place in the Universe." Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights

In one of its earliest possible moments, the universe cracked. These cracks — these tears and rips in space-time itself — may even remain to the present day. An artifact of a long-dead era, an era ruled by exotic forces and strange energies may persist. 

Our universe may be riddled with so-called cosmic strings, these defects in space-time. And while we don't have any evidence yet that they exist, they may still be out there, and I promise that you really don't want to encounter one.

Related: Our Expanding Universe: Age, History & Other Facts

Locked in ice

The Big Bang model of the universe is very simple. A long time ago, the universe was very small, very hot and very dense. At some stage, about 13.8 billion years ago, the universe was so small, so hot and so dense that it was in a completely different fundamental state. The four forces of nature that we know and love today were at one point fused together into a single unified force. As the universe cooled and expanded in its first billionth of a billionth of a billionth (and probably add a few more billionths to that for safe measure) of a second, the forces split off from each other, one by one.

This was a radical phase transition, during which the universe completely changed character. But like all other phase transitions, it may not have been perfect. Or, at least, it may not have happened in the exact same way at the exact same time throughout the universe. To understand this, take a much more mundane and familiar example: an ice cube tray. When you put water in the tray and stick the tray in the freezer, the water eventually reaches a cold enough temperature that it changes phase, from liquid to solid. And this phase transition doesn't happen perfectly throughout the ice cube.

When water switches from liquid to frozen, it starts at what's called a nucleation point. This is some random spot in the soon-to-be ice cube that begins to form the crystal lattice that we call ice, and from that point the phase transition spreads outward to encompass all the water, with the molecules lining up nice and neat and tidy in one particular direction. But that nucleation point doesn't have to be alone. It takes time for that phase transition to spread throughout the water. And in that time, another nucleation point can start forming a crystal lattice all of its own. And that point may very well decide to have a completely different orientation than the first one.

The end result is that your ice cube has defects; it's not perfectly crystal clear throughout its entire volume. And this is apparent to the naked eye. Pick up an ice cube and stare, and you'll see cracks and walls and sometimes even bubbles. These are the leftover imperfections that resulted when two different sets of phase transitions in the crystal lattice didn't exactly align.

So, just take this example and scale it up by, say, a trillion or quadrillion degrees or so, and you have a picture of the early universe.

Related: Big Bang Theory: 5 Weird Facts About Seeing the Universe's Birth

Beautiful flaws

As you might imagine, the defects and cracks left in space-time as the universe changed phases and the forces of nature split off from each other are a bit more weird and exotic than some imperfections in an ice cube. Some of the most common defects left over from this process are the so-called cosmic strings. These are unrelated to the strings of string theory, which are properly called superstrings. (There might be a way to generate cosmic strings from superstrings, but we're not going to get into that right now.)

These cosmic strings stretch from one end of the observable universe to the other and are one-dimensional defects in space-time itself. As the universe grows, the strings grow right along with it, because they are simply a part of the underlying fabric of space. So, as space-time expands, so do the strings.

They're not completely one-dimensional, however. They have a little bit of character. Their width depends on exactly when and how the phase transition occurred, but most theories put them at about the width of a proton. They have no mass in and of themselves; it's not like they're made of something, like you are made of atoms). But since they are wrinkles in space-time, they do have tension, because deformations in space-time are what cause gravitational attraction. This tension makes it seem as if the cosmic strings do have mass.

The mass depends on the density and the level of tension in the string. The resulting figures all depend on the particular theory that's employed, but a good rule of thumb is that 1 inch (2.5 centimeters) of cosmic string has about the same mass as Mount Everest, and 1 mile (1.6 kilometers) will give you the same mass as the planet Earth. Keep in mind that these cosmic strings stretch from one end of the observable universe to the other.

Conditions around these cosmic strings are very strange. Because they bend space and time around themselves, if you were to travel in a circle around one, you would find that you would have to travel less than 360 degrees to return to your starting point. It almost seems impossible, but that's life in curved space-time around cosmic strings.

Related: 8 Baffling Astronomy Mysteries

Put a little wiggle in it

What's more, the strings wiggle. A lot. Sometimes they wiggle so violently that they loop in on themselves, pinching themselves off and sending a closed loop out beyond the remaining string. Sometimes two or more strings will find each other, wiggle a little bit and intersect, cutting themselves off in that way. Sometimes the wiggles become especially violent, forming kinks and cusps that travel up and down the strings at the speed of light.

It's all of these wiggles that render cosmic strings potentially detectable. The closed loops and the cusps generate gravitational waves. These gravitational waves weaken and eventually dissolve the strings. Thankfully this takes a long time, and so if any strings were produced in the early universe, they ought to still be hanging around today.

We haven't detected any gravitational waves from cosmic strings. We might be able to in the future with upgrades to the Laser Interferometer Gravitational-Wave Observatory, or measurements made by Europe's Laser Interferometer Space Antenna spacecraft, which is scheduled to launch in the mid-2030s.

There's another technique, though, that might allow us to see cosmic strings. And that's by, well, looking for them. Light will get split in the vicinity of a cosmic string, so if one happens to just incidentally fall in our line of sight to a distant galaxy, the image of that galaxy will be split in two. We've searched and searched in vain for this evidence, too, but have come up with the same results as with gravitational waves: no signs of any cosmic strings.

If any cosmic strings remain in our universe, they can't have a lot of tension and hence a lot of mass; otherwise we would have seen them by now in some fashion. But cosmic strings are generically predicted by our theories of the phase transitions of the early universe. So either our understanding of cosmic strings is wrong (which is likely to be the case) or our understanding of the early universe is wrong (which is also likely to be the case).

No matter what, if we're able to eventually detect a cosmic string, it will help us unlock some of the deepest mysteries and transformational epochs in our universe. Let's just hope they keep on wiggling.

Learn more by listening to the episode "What are the mysterious cosmic strings?" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to Raul P. for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter. 

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